Recent analyses of the latest series of climate model simulations
suggest that increasing CO2 emissions in the atmosphere
are partly responsible for:
(i) the observed poleward-shifting and strengthening of the Southern Hemisphere
subpolar westerlies (in association with shifting of the Southern Annular Mode toward a higher
index state), and
(ii) the observed warming of the subsurface Southern Ocean.
Here we explore the role that poleward intensifying westerlies play in subsurface
Southern Ocean warming.
To this end a climate model of intermediate complexity was driven separately,
and in combination, with time-varying CO2 emissions and time-varying surface
winds (derived from the fully coupled climate model simulations
mentioned above).
Our experiments suggest that the combination of the direct radiative effect
of CO2 emissions and poleward intensified winds sets the overall magnitude
of Southern Ocean warming, and that the poleward intensified winds are
key in terms of determining its latitudinal structure.
In particular, poleward
intensified winds significantly enhances pure CO2-induced subsurface warming around 45 degrees S,
and reduces it at higher and lower latitudes. Our experiments also support recent
high-resolution ocean model experiments
suggesting that enhanced mesoscale eddy activity associated with poleward
intensified winds influences subsurface warming. In particular, we
find that increased poleward heat transport associated with increased mesoscale
eddy activity enhances the warming south of the Antarctic Circumpolar Current.
Finally, we report on a mechanism involving offshore Ekman sea ice transport
(modulated by enhanced mesoscale activity) which acts to significantly limit the
human-induced high-latitude Southern Hemisphere surface temperature response.

Last Updated: 2007-12-18